Groom's mother toast at rehearsal dinner
Should the groom's mother make a toast at the rehearsal dinner? If so,
Any suggestions on what to say?

Traditionally the Groom's parents host the rehearsal dinner. This would be the appropriate time for the Groom's mother and/or father to make a toast. Keep it short and simple. Thank everyone for coming, welcome the bride and her family to your family and wish the couple well.

No rule about it one way or another. I'd just offer the same advice as for any wedding related toast - avoid the embarrassing child stories about the bride and groom, say nothing off-color or inappropriate or sexual, dn't say anything that would embarrass either of them. Just speak from the heart, welcome your new son/daughter in law to the family, express how proud you are of your child who's marrying - that kind of thing. I think it would be lovely to do, but nobody will expect it so don't feel pressured.